Sometimes you have to change the rendering options, though I don't know if that's true in WC3.
I know in Diablo 2 (a slightly older Blizzard title) you had to use a Glide Wrapper to get Widescreen support. WC3 may be the same way, but I would see if there are other rendering options in the Video Settings of WC3, if what the above poster suggested doesn't work out.